This assumption is a mistake the author is making. Chrome is, for all practical purposes, closed-source and proprietary. It's Chromium that is open-source. So far as I know, Chromium does not have Widevine included by default.
There is no browser support of 4K playback on Amazon at all, irrespective of the DRM situation.
It may have been a stupid decision for W£C to include DRM in the spec, but frankly the rest of the internet doesn't give a shit - As far as OSS is concerned it's another 3rd party blob like flash and not a true part of the web. As far as I can tell the author is trying to use this third party blob specifically - not the rest of the browser for which no one has any authority in the creation of...
So, switched to Firefox. Firefox then also had it break on Debian/Gentoo: https://bugzilla.mozilla.org/show_bug.cgi?id=1475260.
Chromium, and other FOSS browsers including Firefox, do support Widevine. It is a giant proprietary binary blob, yes, but there's support for it. (And yes, I wish everyone would adopt and obey the tenants of free software, but alas.)